1. Field of the Invention
The present invention relates to a method of fabricating high birefringence optical fibers and, more particularly, to a method wherein an optical fiber preform comprising a circular core and circular cladding regions is heated and deformed such that the circular cladding layers obtain a high degree of ellipticity.
2. Description of the Prior Art
Optical waveguides capable of transmitting power with only one direction of polarization are desirable in many different applications, such as fiber sensors, inline fiber devices, Raman lasers, and the like. A variety of structures now exist for introducing birefringence into optical fiber preforms to make polarization-preserving optical fibers. In most of the arrangements, differential stress is introduced in the core by way of an elliptical stress cladding, stress in an elliptical core or isolated stress lobes. Polarization-preserving fiber also utilize geometrical birefringence from a noncircular core or from index side-pits next to the core. These structures are made by a variety of techniques which include grinding the preform before and after deposit and collapse, pressure differential during collapse, rod and tube techniques, or etching.
One method of preform fabrication is disclosed in U.S. Pat. No. 3,982,916 issued to S. E. Miller on Sept. 28, 1976, which relates to a process for manufacturing preforms using the known chemical vapor deposition (CVD) process. As disclosed, the method relates to a process where clad optical fibers having longitudinal, eccentric, azimuthal index homogeneities are drawn. In particular, the CVD process for making preforms is modified by the use of asymmetric heating to produce circumferentially alternating deposits of doped and undoped glass which act as the fiber core when the preform is subsequently collapsed and drawn into a fiber.
A method of fabricating an elliptical core single mode optical fiber is disclosed in U.S. Pat. No. 4,184,859 issued to M. S. Maklad on Jan. 22, 1980. In this process, a multi-layered preform of circular cross-section is subject to partial collapse, first on one side of the preform and then on the other, until an elliptically shaped preform is obtained. The preform is then collapsed to obtain a cylindrical outer shape, where the ellipticity of the core and central cladding layers is maintained. The fiber is then drawn down by any of the known methods, and a round fiber is achieved which includes an elliptical core.
An alternative prior art method of obtaining a fiber which utilizes stress-induced birefringence to obtain single-mode guiding is disclosed in U.S. Pat. No. 4,360,371 issued to M. G. Blankenship et al on Nov. 23, 1982. Here, a hollow intermediate product is formed by depositing layers of cladding and core glass on the inner surface of a substrate tube. Opposite sides of the intermediate product are heated to cause it to collapse into a solid preform foreproduct having an oblong cross-section. A layer of flame hydrolysis-produced soot having a circular outer surface is deposited on the preform foreproduct and is consolidated to form a dense glass cladding layer thereon. The temperature coefficient of expansion of the outer cladding layer is different from that of the preform foreproduct on which it is deposited so that when the resultant preform is drawn into a fiber, a stress-induced birefringence exits in the core.
An alternative method of fabricating a polarization-preserving optical fiber capable of producing a stress-induced birefringence therein is disclosed in U.S. Pat. No. 4,274,854 issued to W. Pleibel et al on June 23, 1981. The method as disclosed has a first step of fabricating a substrate tube to have a wall of nonuniform thickness, the nonuniformity in thickness being arranged about the wall of the substrate tube so that maxima and minima in wall thickness lie in planes which are substantially orthogonal. This first step is followed by deposition of cladding layers and a core layer within the substrate tube. The substrate is then collapsed and the fiber drawn therefrom. The nonuniform wall thickness of the substrate tube translates into elliptical stress cladding, where this cladding operates with differential thermal contraction of the layers to produce stress-induced birefringence in the fiber, which birefringence provides a polarization-preserving optical fiber.
The above-described prior art arrangements all require a large number of fabricating steps, where inconsistencies in the drawn fiber may be introduced at each step, resulting in a drawn fiber which is not as highly birefringent as is necessary for many fiber optic applications. Also, many of the above-described methods are not compatible with existing optical fiber manufacturing facilities and require additional fabrication to produce polarization-preserving fibers.